Restore previous session and Recenly closed windwos under History are grayed out.
Files sessionstore.js and sessionstore.bak cannot be found when I run Search for them. The last winodw that appeared after the crash and tried to restore had this information:

Well, this is embarrassing.
Firefox is having trouble recovering your windows and tabs. This is usually caused by a recently opened web page.
You can try:

Removing one or more tabs that you think may be causing the problem
Starting an entirely new browsing session

But there was no list of tabs in this window and the button Restore was grayed out and there was not an option to start a new session only button Close. I hit Close.

Just before that crash I had two windows with multiple tabs opened.
How can I restore those sessions? Wher can I find the information about them?

Restore previous session and Recenly closed windwos under History are grayed out.
Files sessionstore.js and sessionstore.bak cannot be found when I run Search for them. The last winodw that appeared after the crash and tried to restore had this information:
Well, this is embarrassing.
Firefox is having trouble recovering your windows and tabs. This is usually caused by a recently opened web page.
You can try:
Removing one or more tabs that you think may be causing the problem
Starting an entirely new browsing session
But there was no list of tabs in this window and the button Restore was grayed out and there was not an option to start a new session only button Close. I hit Close.
Just before that crash I had two windows with multiple tabs opened.
How can I restore those sessions? Wher can I find the information about them?

You can check the browser.sessionstore.* prefs on the about:config page and reset user set (bold) browser.sessionstore.*prefs to the default value via the right-click context menu to make sure that Session Restore is enabled and working properly.

If you use cleanup software like CCleaner then check the settings for the Firefox application.

You can check the browser.sessionstore.* prefs on the <b>about:config</b> page and reset user set (bold) browser.sessionstore.*prefs to the default value via the right-click context menu to make sure that Session Restore is enabled and working properly.
*http://kb.mozillazine.org/Session_Restore
*https://support.mozilla.org/kb/restore-previous-session
*http://kb.mozillazine.org/about:config
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If you use cleanup software like CCleaner then check the settings for the Firefox application.

The browser.sessionstore.restore_on_demand pref is the setting in Options > Tabs: Don’t load tabs until selected.

You can use this button to go to the Firefox profile folder:

Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

The browser.sessionstore.restore_on_demand pref is the setting in Options > Tabs: Don’t load tabs until selected.
You can use this button to go to the Firefox profile folder:
*Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

You won't find browser.session.store in the Firefox profile folder, but you can check if you have any sessionstore files.
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You can try to create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
*https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
*http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues
If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.
*http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Helpful Reply

I lost too many saved pages and bookmarks because the Restore Previous Sessioin option disappeared from my home page and was grayed out under history. If you're not computer savvy and the complicated suggested fixes are over your head or don't work, save all your Firefox bookmarks to Google Chrome. Delete Firefox and start using Google Chrome. I have never had this problem with Google Chrome.

I lost too many saved pages and bookmarks because the Restore Previous Sessioin option disappeared from my home page and was grayed out under history. If you're not computer savvy and the complicated suggested fixes are over your head or don't work, save all your Firefox bookmarks to Google Chrome. Delete Firefox and start using Google Chrome. I have never had this problem with Google Chrome.

Just happened to me after a system crash. I back up systemstore.js nightly. I retrieved the latest version but it appears to have been corrupted. Fortunately, the previous version worked. Lesson: Back up systemstore.js routinely and use a backup system that keeps multiple versions. Unfortunately, this won't help you get your session back if you haven't been backing up this folder. (On my Win7 system the file is in C:\Users\[username]\AppData\Roaming\Mozilla\Firefox\Profiles\[folder])

Just happened to me after a system crash. I back up systemstore.js nightly. I retrieved the latest version but it appears to have been corrupted. Fortunately, the previous version worked. Lesson: Back up systemstore.js routinely and use a backup system that keeps multiple versions. Unfortunately, this won't help you get your session back if you haven't been backing up this folder. (On my Win7 system the file is in C:\Users\[username]\AppData\Roaming\Mozilla\Firefox\Profiles\[folder])

If the sessionstore.js file isn't very large then you can try to validate the file online.
*JSON Formatter & Validator: http://jsonformatter.curiousconcept.com/
*JSONLint - The JSON Validator: http://jsonlint.com/